Orthogonal frequency division multiplexing (OFDM) [1] is a special case of multi-carrier transmission, where a single data-stream is transmitted over a number of lower rate sub-carriers. One of the main reasons for using OFDM in recent wireless communication developments is to increase the robustness against frequency selective fading or narrowband interference. On the downlink, OFDM techniques can provide very high-speed downlink packet access by means of a high-speed downlink shared channel based on adaptive modulation and coding (AMC) [2]. However, OFDM requires high signal-to-noise ratio (SNR) in order to provide high-speed packet data transmission.
High-speed downlink packet access (HSDPA) [2] and 1xEV-DV systems [3] based on CDMA techniques have been used to transmit data information. With these schemes, the serving base-station (BS) allocates an active channel (N PN Walsh codes) to a particular served mobile. Such systems, however, do not consider the optimum allocation of Walsh codes for multiple users in each slot.
In the next deployment phase of current cellular systems, such as HSDPA and 1xEV-DV, the frequency assignment is based on the reuse-1, where each sector in each cell utilizes the entire frequency band. This may result in low SNRs for many users, due to the interference from other cells. An OFDM system requires high SNRs to support high-speed data rate transmission.
The frequency reuse-1 scheme is illustrated diagramatically in FIG. 1 where a cell generally indicated by 10 has three sectors 12, 14 and 16. All three of these sectors are operating on the same frequency band “F”.